Ex-weatherman Anthony Watts says many US weather stations produce unreliable data because they are located next to artificial heat – but a scientific analysis suggests that, if anything, such stations underestimate warming

Some amateur climate sceptics have said weather stations across the US are poorly located and thus cannot be relied on. Photograph: Junos/Corbis

It appeared to have shaken the credibility of one of the most important global warming data sets in the world. A blog-inspired campaign by amateur climate sceptics seemed to show that numerous weather stations across the US were so poorly located they could not be relied upon.

The US temperature record uses data from thousands of weather stations spread around the country. Their accuracy was called into question following a campaign by climate sceptic Anthony Watts, an ex-weatherman who runs the influential blog WattsUpWithThat.

He set up a site called surfacestations.org for readers to post photos of poorly located weather stations, particularly in places that could be influenced by artificial heat, such as air conditioning units or car parks. The photos were compiled into a book published by the right-wing thinktank the Heartland Institute. In it, Watts wrote: "The conclusion is inescapable: The US temperature record is unreliable. And since the US record is thought to be 'the best in the world,' it follows that the global database is likely similarly compromised and unreliable."

But scientists at the National Climatic Data Centre (NCDC) in North Carolina have analysed the weather station data to see what difference poor location actually makes. Watts had ranked the stations by his estimation of the quality of their location, so Dr Matthew Menne and colleagues compared the results from high- and low-ranked stations. They described their results as "counterintuitive" – poorly located stations were actually more likely to be cooler than those in better locations. This is probably because the poorly located stations are more likely to use more up-to-date measuring equipment called Maximum-Minimum Temperature System (MMTS), which has a slight "cool" bias that is already well documented.

"NCDC has spent more than 25 years developing methods to correct for biases such as those introduced by the MMTS switch, and published numerous articles documenting these methods and their reliability in the peer-reviewed scientific literature," said Dr Scott Hausman, NCDC's deputy director. "These methods have proven time and again to be robust at removing these biases and providing a true climate signal in regional to global temperatures, and we are highly confident in our results."

Watts challenged this account. He said the NCDC director, Dr Tom Karl, wrote to him in September to offer a "joint scientific inquiry". Watts said he responded nearly two months later to accept the invitation but never received a reply. "The appearance of the Menne et al paper was a bit of a surprise," he said. "By not bothering to reply to our letter about an offer he initiated, and by not giving me any review process opportunity, [Karl] extends professional discourtesy to my own volunteers and my team's work."

He argued that the analysis in the paper is flawed because it only uses preliminary data from 43% of the stations surveyed. But Hausman said he did not believe this would affect the results. "We tested the sensitivity of our results to this and found the differences to be insignificant," he said.

Watts said he is now producing an analysis using data from 87% of the stations.